A serious difficulty of formation pressure determination during drilling operations is related to the pressure build-up around a wellbore exposed to overbalanced pressure and subject to filtrate leak-off called supercharging. This pressure build-up is accompanied by filter cake deposition and growth externally, at the sand face, and internally due to the mud filtrate invasion. Thus, the filter cake hydraulic conductivity changes with time, affecting the pressure drop across it and therefore the pressure behind it, at the sand face. This makes it difficult to predict the evolution of the pressure profile with time, even if the history of local wellbore pressure variation has been recorded.
Existing formation pressure measurements, made with so-called formation testing tools which probe the formations, often read high compared to the actual reservoir pressure far from the borehole, due to the supercharging effect. There are currently no known commercially viable techniques for the determination of the formation pressure in relatively low permeability reservoirs (below approximately 1 mD/cp) during drilling operations which adequately account for supercharging. The main difficulties are related to (1) the poor filter cake property, (2) the long actual time of wellbore exposure to overbalanced pressure, and (3) the practical time constraints, which require the pressure measurements to be carried out during a rather short time compared to the time of pressure build-up around a wellbore. These constraints make it difficult, if not impossible, to sense the far field formation pressure, at the boundary of the pressure build-up zone, with the usual transient pressure testing techniques, because of the slow pressure wave propagation inherent in low permeability formations.
Accordingly, while existing tools and techniques can often work well in relatively high permeability formations, where supercharging easily dissipates, e.g. during tool setting, there is a need for a technique that can be successfully employed in relatively low permeability formations. It is further desirable to have a technique that is applicable to formations of wide ranging permeability, irrespective of the origin of the supercharging. There is also a need for accurate determination of filtrate leak-off parameters. It is among the objects of the present invention to address these needs.